[hn_throwaway_99]

No they didn't. A control group inherently implies the researchers made an intervention in the first place (or else there was a "natural experiment" where people were randomly placed in group or another). That didn't happen here, as this was purely an observational study.

[tmule]

Observational studies can have synthetic controls. The term, as such, doesn’t mean precisely what you think it means.

[hn_throwaway_99]

I understand that. But saying "they had a control" in any case implies an "intervention" group of some sort, whether synthetic or natural, and this is very different from saying "they controlled for other factors", which is what they did in this study.

The study authors aren't even purporting to show a casual relationship (i.e. that improving sleep would necessarily reduce mortality), just that irregular sleep predicts mortality.